The Effects of Geometric Parameters under Small and Large Deformations on Dissipative Performance of Shape Memory Alloy Helical Springs

This paper presents an investigation into shape memory alloy (SMA) springs considering the effects of geometry changes under small as well as large deformations. Helical springs are fabricated by shape setting of NiTi wires through heat treatment followed by cooling in furnace. The products exhibit pseudoelasticity at the ambient temperature, and their force-displacement responses are examined by conducting simple tension test. A model is further proposed to study tension and compression of SMA springs, and it is shown that the consequences of geometrical changes in tension and compression of springs are different. Under tension, at early stages of loading, the assumption of small deformations can reasonably predict the response of an SMA spring; however, the error of using small deformation assumption increases with increase in the amount of applied force. Unlike tension, in compression, small deformation model predicts the response of SMA spring with a considerable error even from the beginning of loading. The numerical results of large and small deformation models are verified by experimental findings. In order to design a spring having maximum dissipative performance, a designer has three geometric parameters to set: wire diameter, spring diameter and the number of active coils. The influences of these parameters on dissipated energy are investigated in both displacement- and force-control loadings, and a framework for designing SMA springs with the purpose of achieving maximum applicable dissipation is at last developed.